This application is the national phase under 35 U.S.C. xc2xa7 371 of PCT International Application No. PCT/FI99/00354 which has an International filing date of Apr. 28, 1999, which designated the United States of America.
1. Field of the Invention
The present invention relates to a housing for a fused switch.
2. Description of Conventional Art
Fused switches having a fused switch housing of the type concerned herein are used, e.g., in electricity distribution, supply of electric equipment and in different types of electric circuits. The function of a fused switch is to pass electric current from a feeder circuit to a consumer circuit via at least one fuse located in the fused switch. Another function of the fused switch is to serve as a circuit breaker between the feeder and consumer circuits permitting the electric connection between said circuits to be interrupted when so required. A single fused switch can be made to connect a plurality of electric circuits to each other. For instance, a three-phase fused switch may comprise at least three first and second contact members with conductor parts connecting said members via fuses.
Conventionally, a fused switch includes at least one pair of moving contacts and a fuse connected in series. In modern fused switches the fuses are mounted in removable fuse adapters in order to ease the task of fuse replacement. This kind of switch design with a fuse adapter facilitates the use of single fused switch housing in conjunction with a number of different fuse types. Thence, only the fuse adapter need to be selected compatible with the fuse type in use. In a well-designed adapter-type fused switch, fuse replacement is also extremely safe to the person performing the operation. The fuse adapter comprises contact parts on which the fuse is easy to mount. The contact parts of the fuse adapter also act as the electrical contact between the fuse and the elements of the fused switch housing when the adapter with the fuse is inserted in place. The housing of the fused switch carries the stationary contacts with their springed contact parts serving to form an electrical contact with the mating contact parts of the fuse adapter. In the switched circuit, the stationary contacts of the switch housing located to both sides of the fuse are arranged to cooperate either on both sides with their respective moving contacts, or alternatively, so arranged that on one side the stationary contacts cooperate with the moving contacts and on the other side are directly connected to the terminals of the external circuit.
The path of current flowing through a conventional fused switch passes from the input terminals of the fused switch over the moving contact members and their contact surfaces to the stationary contacts of the switch housing, and therefrom, further over other connections to the first contact parts of the fuse adapter. Subsequent to the fuse adapter contact parts, current passes over the interface between these contact parts and those of the fuse proper, and then over the second, similar fuse contact interface to the second contact parts of the fuse adapter. Prior to reaching the output terminals of the fused switch, the current must pass at least the contact interface between the second contact parts of the fuse adapter and the second stationary contact parts of the fused switch. Resultingly, each current path of conventional fused switches contains at least four connections/contacts on the fuse and switch side of the fuse adapter plus the internal connections of the switch itself.
The number and quality of connections in a fused switch affect the properties of the fused switch. This is because each connection involves a separate interface resistance that in turn causes additional heating of the fused switch and thus reduces its load rating. Furthermore, the fabrication of any single connection is a separate cost factor and each connection is a potential origin of malfunction. Another aspect affecting the current breaking capacity of a switch is essentially related to the number and operating speed of current-breaking connections provided on the current path. The greater the number of simultaneously operating air gaps in series the faster the switch can quench a possible arc striking between the contact surfaces of opening switch members. Arcing causes undesirable wear in the switch and generates interference in the form of electromagnetic radiation, for instance. The quenching properties of the contact gap arc are particularly important in DC switches. AC switches are not so critical in this sense inasmuch the current passing in an AC circuit over an arc inherently crosses zero twice during each cycle.
Hence, the type and number of contacts acting as opening gaps in a fused switch must be designed chiefly on the basis of their current-breaking properties. In turn, the number of connections not participating in current turn-off should be minimized.
Prior-art technology of fuse-adapter-type fused switches is handicapped by having at least four connections per each current path suck that they do not participate in the actual switching operation.
It is an object of the invention to provide an entirely new type of housing for a fused switch, wherein redundant connections are eliminated.
The goal of the invention is achieved by a switch design having at least some of the fuse adapter contact surfaces which face those of the switch housing and make the electrical connections between the fuse and the switch housing to additionally perform as opening/closing contacts of the switch. Accordingly, the invention is based on combining at least one of two connections which make the electrical connections between the fuse and the switch housing with its series-connected opening/closing contact so as to form a single contact-type connection.
The invention offers significant benefits.
With the exception of fuse connections, the fused switch according to the invention need not necessarily have any such internal connections that do not participate with the circuit switching functions. The connections between the fuses and contact parts of the fuse adapter are required for implementing the adapter-type fused switch design. Hence, with the aid of the invention, unnecessary connections in the present fused switch construction are disposed of.